Generally, a rechargeable battery is mounted to an electric car or a portable electronic device such as a video camera, a mobile phone, a portable PC, PMP and a MP3 player, so the portable electronic device or the electric car is operated using the power output from the battery.
In case a battery is used for a portable electronic device, the battery is recharged using a separate charging unit. In case a battery is used for an electric car, the battery is recharged using electricity generated by a rotational force of a driving wheel while the car is running.
Meanwhile, if a battery is connected to a load to initiate discharging, a terminal voltage between an anode and a cathode is gradually decreased. Also, if the terminal voltage reaches a certain limit, the terminal voltage between the cathode and the anode is rapidly lowered to a discharge end voltage, and after that, the discharging capability is lost. If the battery is discharged below the discharge end voltage, chemical reaction is caused between an electrolyte and an electrode plate, thereby damaging the electrode plate that generates electric current. If this phenomenon is repeated, the battery may lose its function or suffer from a shortened life cycle.
Thus, when using a battery, it is very important to charge the battery at a suitable point by exactly checking SOC (State Of Charge) of the battery and also to prevent the battery from being discharged to a discharge end voltage. In particular, in case of an electric car or a hybrid car, SOC of a battery should be accurately checked so as to prevent in advance the car from abruptly stopping while running and also to stably supply power to a motor of the car. However, since the battery characteristic is changed non-linearly (for example, varying according to temperature, discharge or the like), it is difficult to exactly measure SOC of the battery.
There have been frequently used a method for estimating SOC of a battery according to OCV (Open Circuit Voltage) of the battery and a method for estimating SOC of a battery based on a discharged current of the battery.
In the former method, a varying pattern of OCV is prepared in the form of a look-up table for each SOC in advance, and then a SOC is mapped from the look-up table according to an output voltage of the battery. However, in the step of charging or discharging the battery, there occurs an error between an actual voltage and a measured voltage due to IR drop, so it is not accurate to estimate SOC of a battery based on a battery voltage.
For reference, IR drop means that a voltage is rapidly changed when a battery is connected to a load and initiates charging or discharging. That is to say, the battery voltage is rapidly lowered when the battery initiates discharging, while the voltage is rapidly increased when the battery initiates charging.
In the latter method, SOC is estimated through a current load of the battery, namely a current capacity calculated by accumulating currents flowing toward the load. However, this method exhibits deteriorated accuracy in estimation of SOC as time goes since a measurement error occurring at measurement of electric current is also accumulated.
Accordingly, in recent days, various methods for correcting SOC of a battery have been suggested to solve the above conventional problems. Such methods adopt complicated mathematical models, which however require a lot of time for estimation of SOC of a battery and also need high-tech hardware for executing complicated operations.